1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a system and method for interchanging and rearranging cluster assemblies of fresh and burnt fuel within fuel assemblies containing fresh and burnt fuel to improve fuel productivity.
2. Description of the Prior Art
A typical pressurized water nuclear reactor contains a large number of fuel assemblies in its core. Each fuel assembly is substantially identical to the next except for the fuel enrichment of the individual assembly. In order to optimize the fuel burnup and smooth the radial neutron flux profile across the reactor core, historically a zoned enrichment pattern has been used. Particularly, fuel contained in assemblies located in regions nearer to the periphery of the core is somewhat more enriched than fuel contained in assemblies located in regions nearer to the center of the core. After a given core cycle, such as a year, fuel assemblies in a higher enrichment core region are shuffled into a lower enrichment region, while new fuel assemblies are added to the highest enrichment region and depleted or burned out assemblies are removed from the lowest enrichment region.
Notwithstanding their variation in fuel enrichment, all of the fuel assemblies in the reactor core have the same construction. Basically, each fuel assembly is composed of a bottom nozzle, a top nozzle, an instrumentation tube and pluralities of guide thimbles, fuel rods and grids. For instance, in one exemplary fuel assembly, the fuel rods are arranged in a square 17 by 17 array with 17 rod locations per side. Of the total possible 289 rod locations per assembly, 264 locations contain fuel rods. In addition to the single bottom nozzle, top nozzle and instrumentation tube, there are 24 guide thimbles and 8 grids.
The structural skeleton of the fuel assembly is composed of the bottom and top nozzles and the plurality of guide thimbles which extend vertically between the bottom and top nozzles and rigidly interconnect them. In addition to their shared function of providing the fuel assembly with a rigid skeleton, each one serves other functions. The bottom nozzle directs the distribution of upward coolant flow to the fuel assembly. The guide thimbles provide channels through the fuel assembly for insertion of control-type rods therein. The top nozzle provides a partial support platform for the spider assembly mounting the respective control rods. The top nozzle also has openings which permit upward flow of coolant through it. Also, the bottom and top nozzles respectively act to prevent either downward or upward ejection of a fuel rod from the fuel assembly.
The grids and fuel rods are not structural parts of the fuel assembly but instead are respectively supported directly and indirectly by the guide thimbles. The grids are attached in axially spaced positions along the guide thimbles such that the multiplicity of cells defined by interleaved straps of the respective grids are disposed in vertical alignment. The fuel rods are supported in an organized and transversely spaced array in the vertically aligned cells of the transverse grids by springs and dimples on the straps which extend into the cells. Each fuel rod contains nuclear fuel pellets and the opposite ends of the rod being closed by upper and lower end plugs are spaced below the top nozzle and above the bottom nozzle. The fuel pellets composed of fissile material are responsible for creating the reactive power of the reactor which is transferred in the form of heat energy to coolant flowing upwardly through the fuel assembly.
The guide thimbles are larger in diameter than the fuel rods and, as mentioned above, provide channels adapted to accommodate various types of control rods used in controlling the reactivity of the nuclear fuel. A more detailed description of this typical fuel assembly and the types of rods insertable in the guide thimbles thereof may be gained from U. S. Pat. No. 4,432,934 to Robert K. Gjertsen et al, which patent is assigned to the assignee of the present invention. While the guide thimbles accommodate various types of control rods, fuel rods cannot be placed in them when they are not being otherwise used since the fuel rods would overheat due to lack of enough remaining space within the guide thimble to accommodate sufficient coolant to carry the heat away.
Thus, the conventional fuel assembly has a significant number of its rod locations, approaching ten percent in the example above, dedicated to nonfuel use. Even more, since not all fuel assemblies in the reactor core require control rods (about two-thirds of the fuel assemblies in a typical core do not), nonfuel rod locations in many regions of the core go unused which results in reduced power output, increased fuel cycle costs, reduced fuel assembly life and a suboptimum fuel loading configuration. Also, while the interconnections provided by the guide thimbles provide a rigid skeleton of high structural integrity, the large number of thimbles increases the difficulty of top nozzle removal and remounting in carrying out fuel assembly reconstitution.
Consequently, a need exists for a fresh approach to fuel assembly design which would avoid or reduce some of the limitations and shortcomings inherent in the conventional fuel assembly construction described above and enhance its adaptability without sacrificing its structural integrity.